An estimated 500,000 people around the world die each year from cancer of the colon or large bowel. Experts say most of those deaths could be prevented with improved screening methods. A new non-invasive test is being developed by British researchers to identify people with colon cancer.

The old tried-and-true methods for detecting colon cancer is the so-called fecal occult test, in which laboratory technicians look for the presence of blood in stool samples. The problem with this method is there is a high rate of so-called "false positives," suggesting the presence of cancerous polyps where none exists.

Then there are the visual scopes that doctors use to look inside the colon for pre-cancerous or cancerous polyps. But these types of examinations are not only expensive, they must be performed at established medical facilities - a fact that excludes most of the world's population.

For these reasons, researchers at Britain's Medical Research Council Cancer Cell Unit in Cambridge have been working on another detection method. Lead scientist Nicholas Coleman says the work focuses on a protein that is used by cells to reproduce their genetic material.

"The difference between normal tissues and cancerous ones is that in normal tissues, these proteins are present in very restricted regions whereas in cancerous ones there is very abundant expression in parts of the tissue that would not normally contain the markers," he said. "So, for example in the case of the colon, the proteins are only present at the deep aspect at the lining of the bowel whereas in cancer, they become detectable at the very surface layers of the bowel: in other words, those layers which are in contact with the stool as it passes through the bowel."

In a study of forty patients known to have colon cancer, Dr. Coleman says researchers were able to detect the protein, called MCM2, in the stool samples of 37 patients. The protein was not detected at all in a comparison group of 25 healthy people.

Among the cancer cases that were picked up, according to Dr. Coleman, were a substantial number of early cancers, a stage at which the disease is thought to be most treatable. Dr. Coleman says the three cases that were not detected were on the right side of the bowel, where colon cancer is typically the hardest to detect.

Dr. Coleman thinks the new test could be introduced into many parts of the world.

"It offers the potential for being introduced in parts of the world where endoscopy, sigmoidoscopy or colonsocopy, is prohibitively expensive," he said. "We would hope our test would be affordable."

Dr. Coleman also envisions automating aspects of the tests - such as obtaining cells from stool samples and looking for the presence or absence of the protein marker - as a way to keep costs down.

"What they're doing has promise," said David Beck, chairman of colon and rectal surgeon at the Ochsner Clinic Foundation in New Orleans, Louisiana. "It just needs some more evaluation and maybe some improvements."

Before the test could be used in the developing world, Dr. Beck says, British researchers need to overcome two obstacles - the fact that stool samples must be refrigerated, and must be processed within eight hours.

"We have a good test now - colonoscopy - but it's very expensive and invasive," he said. "And so this type of thing is needed if we identify all the patients who are at risk. And the method that they describe may be a way that works. But we need to work out some details outside the research area. In other words, it's kind of different to ask patients to ask you to give you a sample outside in your hospital and you know it, as opposed to trying to expand this out to the whole population."

The work on the colon cancer protein is published in the current issue of The Lancet.